Tea Party and other conservatives had hoped to make their gathering in Washington on Wednesday a clarion call to rally against the Iran nuclear deal, drawing on the star power of Donald J. Trump, the rhetoric of Senator Ted Cruz and a cast of 40 or so supporting characters speaking on a stage with the United States Capitol looming in the background.

But Tuesday, President Obama secured the votes needed to make his negotiated deal with Iran essentially veto proof, giving this rally a less tangible, or at least achievable, objective.

But, as with all great political theater, the show had to go on.

So Mr. Trump, Mr. Cruz, Sarah Palin, the radio host Mark Levin and a host of other conservative luminaries headed outside to brave the swampy September humidity to air their grievances against the deal, committed votes be damned, basking in the lights, camera and attention brought by Mr. Trump.

The businessman arrived well ahead of his speaking slot, and as he lingered, cameras and reporters swarmed, a large hive collecting around him, as Mr. Cruz prepared to take the stage. Mr. Trump bantered over questions about his presence at the rally, the Iran deal and his candidacy, speaking in his customary sound bites.

“There is a little romance between us,” he told the reporters, referring to Mr. Cruz, as the Texas senator took the stage.

Mr. Cruz delivered a blistering critique of the Iran deal, accusing supporters of having their names “on the bomb,” saying “you cannot wash your hands of that blood.” And he called on Republican leadership to use congressional maneuvering to block movement on the Iran deal, with a touch of theatrics.

“I come not to bury Caesar, but to praise him,” Mr. Cruz said, trying to calm the crowd after it began booing at the mention of Speaker John A. Boehner and the Senate majority leader, Mitch McConnell.

As Mr. Cruz finished, Mr. Trump took the stage, and the two embraced in a powerful handshake, fist to chest, before waving to the crowd, stopping short of taking a bow.

“When Donald arrives at an event, he brings an army of TV reporters,” Mr. Cruz said recently, according to CNN. “He brings an army of cameras that show up. And Donald’s being there — he very graciously accepted — means the mainstream media will cover the event.”

It is under the glare of those cameras that Mr. Trump shines, often offering more entertaining and meandering speeches while rarely pausing for a dull moment to talk detailed policy. His speeches make for great proclamations – like the promise to build a wall across the southern border with Mexico, which he would make Mexico pay for.

“I’ve been doing deals for a long time, I’ve been making lots of wonderful deals — great deals — that’s what I do,” Mr. Trump said in his speech. “Never, ever, ever in my life have I seen any transaction as incompetently negotiated as our deal with Iran.”

And so far, Mr. Trump’s style seems to be resonating, even seeping into other candidate’s stump speeches. Mr. Cruz, for his part, calls to “abolish the I.R.S.,” often putting the emphasis on the verb.

Much of the early part of the Republican contest has centered on attention generated by Mr. Trump. Carly Fiorina and Rick Perry have tried to raise their profile, and polling numbers, by attacking Mr. Trump. Jeb Bush has increasingly waged a full-throated campaign against him, while Gov. Chris Christie of New Jersey has been more cordial.

But none have gone further than Mr. Cruz, who has constantly defended Mr. Trump, no matter how caustic his remarks. It was Mr. Cruz who invited Mr. Trump to the rally, not the Tea Party Patriots, the original hosts of the event, and the joint appearance by the two candidates here, Iran votes or not, is a symbolic high point in their relationship so far.

But Mr. Cruz was also willing to lay bare the rationale behind the allegiance. While noting that he likes Mr. Trump, he emphatically stated that he doesn’t believe Mr. Trump is going to be the Republican nominee because Mr. Cruz is running to win himself.

“It is my hope to earn the support of every one of Donald’s supporters,” Mr. Cruz told reporters after he spoke. “Reporters act as if this is, like, a shocking discovery, ‘Cruz is trying to get Trump supporters to support him.’ Of course I am! I’m trying to get the supporters of every other candidate.”